1. Field of the Invention
The present invention relates to a method of cooling section steel, particularly sectional steel girders, from rolling heat.
2. Description of the Related Art
Cooling of section steel, such as sectional steel girders, for example, double T sections and U sections, angle steel, T sections, after rolling is conventionally carried out by means of a cooling bed. Because of the uncontrolled, frequently unfavorable free cooling of the sectional girders or rods during the time when the girders or rods are on the cooling bed, a disadvantageous influence on the straightness and the inherent stress condition is unavoidable in most cases. This is because there is a close causal relationship between the straightness or inherent shape and the inherent stress condition.
Taken together, the two quality criteria mentioned above with respect to sectional girders can be compared to the planeness in the case of strip rolling. However, while the significance of a good planeness is to be seen primarily under geometrical aspects in the case of strips, the length differences of the fibers over the cross-section in the case of comparatively stiff sections may only result in a curvature, but will with certainty result in a sometimes significant reduction of the load carrying capacity due to inherent stresses.
In addition to a reduced load carrying capacity when external loads act on the steel, structural components with inherent stress also are subject to greater distortions during further processing because of the resulting interference with the state of equilibrium and also have a greater tendency to form cracks in areas with great differences in inherent stress, as they may occur particularly in the transition area between a web and a flange, for example, in double T sections.
The present invention is based on the following considerations and findings concerning the mechanism of the creation of inherent stresses. A rolled sectional girder leaves the last roll stand with a good approximation of a homogenous elongation distribution; this means that the girder or rod is straight and has no areas of waviness. In the case of dynamically recrystallizing materials, the girder or rod is essentially free of inherent stresses because of the high temperature level. On the other hand, in the case of a suppressed dynamic recrystallization, which is an important prerequisite for thermomechanical rolling, the inherent stress situation which is characteristic for the last pass reductions is established.
The temperature distribution after the last rolling is usually distinctly inhomogeneous; especially at the locations with material concentrations, a section cools to a lesser extent than in the areas having thin walls. Independently of the thermal initial condition, a section generally cools inhomogenously in air. The resulting different thermal length changes must be compensated by elastic or even elastic/plastic elongations, accompanied by the formation of stresses which occur as an inevitable result. The higher the temperature, the more quickly the stresses of this type are reduced by relaxation, i.e., by a process which is comparable to a concurrently occurring stress relief heat treatment. However, since this takes place more slowly than the thermal changes, internal stresses also act on the section during this phase of high temperatures. In the case of asymmetrical cooling conditions or section geometries, the section or rod assumes a shape because of the occurring distortion in which the inner moment becomes zero unless prevented therefrom by external forces, for example, weight forces, frictional forces or other holding forces, for example, as a result of a straightening grate.
If a fiber or a portion of the section is within the range of gamma-alpha structural transition, all stresses are canceled in this area because of the complete restructuring of the structure. The growth of this fiber caused by the lower packing density of the alpha iron is also partially suppressed because the other fibers which have not yet been transformed because of their residual elasticity resist against the growth of the fiber. In this phase of successively reaching the transformation range, the curvature of a section which asymmetrical or is cooled asymmetrically and is not guided in a straightening grate or other means continuously changes. Only toward the end of the transformation the section is essentially free of inherent stresses and is independent of the freely forming or forced state of curvature. However, when at least two fibers or partial areas have dropped below the lower limit temperature of the transformation, a constraint can again occur between these fibers which is the result of the elastic or elastic/plastic compensation of different thermally caused contractions. The stresses, later the inherent stresses, are essentially pressed below the transformation because of the relaxation which then becomes increasingly insignificant. As cooling progresses, more and more fibers drop out of the range of transformation and participate in the above-described formation of inherent stresses.